The present invention relates generally to a method and apparatus for studying fracture behavior of materials and structures, and more particularly a method and apparatus for generating a measurable natural crack having predictable geometry, location and orientation into a solid material.
Fracture behavior studies of materials and structures include determinations and/or measurements of fracture toughness; structural tests with controlled defects simulating natural defects in structures including, but not limited to, critical shipping containers, aircraft parts, and ship structures; moving cracks in prestressed structures or materials; and crack arrest behavior of materials and structures, e.g., investigating the conditions under which a moving crack will stop.
Prior to the 1940's, fracture safe design procedures for materials were essentially nonexistent. It wasn't until catastrophic failures of many of the World War II "Liberty Ships" occurred that the problem of improved fracture characteristics of metals was finally brought to attention. Fracture initiation in even a minute element of the structure could result in nearly instantaneous fracture of the ship's hull. To this day, the study of fracture initiation and fracture toughness of a material or structure is vital for design criteria.
Fracture resistance or fracture toughness is the inherent resistance of a material to propagation of a pre-existing crack. It is a mechanical property which is very sensitive to the composition, microstructure and prior thermomechanical treatment of the material. All engineering structures have defects (pre-existing cracks) at which fracture will initiate when the stress intensity at the crack tip exceeds a critical value. The stress intensity at the crack tip is determined by the loading on the structure (stress) and the crack size, but the critical value (the stress intensity above which the crack will enlarge at a catastrophic rate) is independent of the engineering design or the loading--i.e., it is a property of the material itself. Therefore, to prevent fracture, the stress intensity must be kept below the critical level, and it is equally important that (1) the stresses are within specified limits, (2) the defects are within tolerable sizes, and (3) the material has adequate fracture toughness (K.sub.IC).
The standard ASTM method for evaluating plane strain fracture toughness generally requires preliminary insertion of low stress fatigue cracks in a notch formed in the material specimen. With early efforts to characterize better fracture, cracks were simulated with sharply-tipped notches. No matter how sharp the notch was machined, however, a natural fatigue crack would often result in still lower values of the fracture toughness. Because pre-existing cracks rather than notches are the usual source of difficulty with engineering structures, the fatigue crack was specified as mandatory for valid tests of fracture toughness K.sub.IC (e.g., see ASTM test method E-399).
Conventional methods of cracking are very slow and costly, and are designed typically for one specimen at a time. Consider the growth rate of 2 micro-inches per cycle. Approximately 50,000 cycles are required for 1/10" of crack growth, typically 25 minutes at 2,000/minute. At these rates, it is not unusual to average about 60 minutes per specimen when allowance for set-up time, crack initiation from the notch, and shut-down for intermittent examinations are considered. Additionally, high capital equipment costs of this method are undesirable.
An apparatus for measuring fracture toughness of a specimen material is disclosed in U.S. Pat. No. 4,075,884, dated Feb. 28, 1978, to Barker. The fracture specimen loading machine apparatus includes a load-generating portion at a pressure bag adapted to be disposed within a flawed portion of the specimen material. Hydraulic fluid is introduced into the pressure bag, causing an expansion which effectively loads the specimen. A pressure sensor is included for measuring the hydraulic fluid pressure within the pressure bag. The hydraulic fluid creates either a sustained pressure within the notch of the specimen, or alternatively is applied in cyclic form. In either case, a natural crack is not formed; rather, the specimen undergoes plastic deformation in the vicinity of the crack tip, and the material's fracture toughness is altered.
It is desirable to provide a method and apparatus for studying fracture behavior of materials and structures which is useful to study and/or determine fracture toughness; structural tests with controlled defects simulating natural defects; moving cracks in prestressed materials or structures; and crack arrest behavior in materials and structure. More particuarly, it is desirable to create a measurable natural crack in a structure or material without initiating significant plastic deformation. In other words, it would be desirable to create a natural crack possessing predictable geometry, location and orientation without administering a sustained shock pulse, or a series of cyclic pulses to a solid material or structure.